Semiconductor laser diodes of the type mentioned above have, for example, become important components in the technology of optical communication, particularly because such laser diodes can be used for amplifying optical signals immediately by optical means, thereby avoiding any complicated conversion of transmitted signals along the route between the transmitter and receiver, and thus improving speed and reliability within such systems.
In one specific kind of optical fiber communications system, laser diodes are used for pumping Erbium-doped fiber amplifiers, so-called EDFAs, which have been described in various patents and publications known to the person skilled in the art. Of some technical significance are ridge-waveguide laser diodes with a power output of 150 mW or more, whose wavelengths match the erbium absorption lines and provide low-noise amplification. Several laser diodes have been found to serve this purpose well and are used today in significant numbers.
One of the major problems of semiconductor laser diodes of the types mentioned above is degradation of the waveguide adjacent to a facet, in particular at the front facet of the laser. This degradation is believed to be caused by temperature increase at the mirror facet regions, especially at high power outputs, which temperature increase in turn is probably caused by unwanted carrier recombination in these regions and heating due to free carrier injection.
This is further aggravated by crystal inhomogeneity—often produced by cleaving the laser cavity—and high optical power densities within these regions, especially in the front facet/mirror vicinity. Again, this affects mainly the regions adjacent to the mirrors which are thus the weak points, in terms of reliability.
Particularly in the front facet region, an increased failure rate at high optical output powers has been observed. The same was found, though to a lesser degree, in the vicinity of the back mirror. Since the material degradation in the facet regions is accelerated by a combination of optical power density and electrical current density, it seems advantageous to reduce the latter.
Consequently, ways have been sought to prevent this carrier recombination in the laser diode's facet regions. Such approaches typically comprise providing current blocking regions above the ridge of a ridge waveguide laser, adjacent to the facet.
Schmidt et al. U.S. Pat. No. 6,782,024, assigned to the assignee of the present invention and incorporated herein by reference, is an example of a solution to the above-identified problem. It discloses a semiconductor ridge-waveguide laser diode with means for limiting the injection of carriers at one or both of the diode's end sections, thus providing at least one “unpumped” end section. This injection limiting means comprises an isolation layer which extends at least partially over the end sections and inhibits the injection of carriers from the usual metallization into the active region of the laser diode. Other examples of such approaches are given by Yu et al. in U.S. Pat. No. 6,373,875 and Sagawa et al. in U.S. Pat. No. 5,844,931.
Manufacturing laser diodes according to the designs described above requires processing steps additional to those required to fabricate a basic device in which the whole ridge is covered with an electrode. Such additional processing steps increase the manufacturing complexity and consequently the price of the product. Further, it appears that the above-identified approaches for reducing the current flow through the end sections of the laser diode's ridge, whether by placing an insulating stripe over the end of the ridge as in U.S. Pat. No. 6,782,024 or by patterning an electrode such that it does not extend over the end section close to the facet as in U.S. Pat. No. 6,373,875 or U.S. Pat. No. 5,844,931, above, do not completely prevent current from spreading into the end sections.
Thus, there is a need for a simple and reliable design for a high power ridge waveguide laser diode which does not suffer from end section degradation.
In making such a laser diode there is also a need to avoid adding to the complexity of the laser diode structure and to keep the number of additional structural components or manufacturing steps of the laser diode at a minimum. Such a manufacturing method should be more economical than those described in the prior art, allowing reliable mass production of ridge waveguide laser diodes.